code: def test_expression_getitem():
      x = datum.foo[0]
      assert repr(x) == "datum.foo[0]"
  
file_path: altair/tests/expr/test_expr.py
imports: import operator
  import pytest
  from altair import expr
  from altair import datum
  from altair import ExprRef
  from jsonschema.exceptions import ValidationError
label_desc: Use image node_id 10 for calling a global function with example usage: test_expression_getitem() without return types